This application claims the priority of German patent document 100 21 907.1, filed May 5, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system having a plurality of fuel cells arranged in a stack between two end plates.
U.S. Pat. No. 5,714,874 discloses a fuel cell system in which a fuel cell stack is connected to an electrical load via a load circuit. Furthermore, a diode is provided in the load circuit between the fuel cell stack and the electrical load. In addition, a current transformer and a battery are integrated in the load circuit. The components in the load circuit are connected in series, so that the order of the components is unimportant.
The diode in the load circuit allows current to flow from the fuel cell stack to the electrical load, while preventing a reverse current from the electrical load into the fuel cell stack. A current flow in the direction opposite the intended flow direction of the fuel cell of this type (i.e., from the anode of the fuel cell stack to the cathode) would cause permanent damage to the fuel cell stack. One example of an electrical load which at the same time may also act as a generator is a converter for a driving motor in a fuel cell vehicle. In a braking mode (i.e., when the vehicle is being decelerated by the drive), a drive converter of this type can itself become a generator, and thus feed current into the load circuit.
The diode is usually electrically connected to the fuel cell stack via an electrical conductor, for example a busbar or a cable. This electrical conductor and the associated connecting element (i.e., a cable lug or plug) must be designed for the high fuel cell currents, and are therefore complex and expensive.
Furthermore, when current flows in the conducting direction in the pn junction of a diode, a voltage always remains in the diode. This forward voltage multiplied by the current passing through the diode results in a power loss which heats the diode. To prevent unacceptable temperatures from being reached in the diode, causing the latter to be destroyed, it is therefore necessary to provide cooling. This cooling may be realized by a liquid coolant or by means of air cooling. In the embodiment with liquid coolant, it must be ensured that a conductive connection is not produced between the anode and the cathode of the diode by the coolant, since otherwise the effect of the diode would be eliminated. In the air-cooled embodiment, a correspondingly large air heat sink has to be used. Under certain circumstances, to achieve sufficient dissipation of heat, forced ventilation by means of a fan is also required. When using ventilation, it is also necessary to ensure that dirt particles are not sucked in, which could combine with moisture present in the air, causing insulation problems on the intended leakage paths.
It is an object of the invention to provide a fuel cell system which is of simple design, functions reliably and has a load circuit in which a fuel cell stack, a diode and an electrical load are arranged.
This and other objects and advantages are achieved by the fuel cell system according to the invention, in which the diode is arranged on or in one of the end plates of the fuel cell stack. At the same time, a cooling device, which is in thermal contact with the diode, is provided in the end plate in the region of the diode. As a result, it is possible to dispense with an additional connection between diode and fuel cell stack, achieving a significant weight, cost, space and design advantage on account of the line cross sections which are required for the high fuel cell currents. The direct electrical connection of the diode to the end plates eliminates the need for electrical contact points, which may lead to contact and insulation problems.
At the same time, it is easy to ensure sufficient cooling of the diode, since a cooling device is usually already present in a fuel cell stack. Therefore, the cooling device which is present needs at most to be expanded slightly. Alternatively, the diode may also be arranged in the region of the cooling device which is already present. It is thus possible to dispense with a separate cooling device for the diode, which likewise constitutes a significant advantage in terms of weight, costs, space and design.
The diode may, for example, be in the form of a disc and is preferably releasably connected to the end plate with the aid of an attachment element. This simplifies installation and, if necessary, exchanging of the diode. The attachment element may also be releasably connected to the load circuit. In this way, it is possible for the attachment element at the same time to be used to bring one pole of the diode into intimate contact with the end plate and, at the same time, to allow simple connection of the load circuit to the other pole of the diode.
Arranging a thermally and/or electrically conductive intermediate layer between diode and end plate allows the thermal and/or electrical conductivity to be improved.
Integrating the semiconductor module of the diode directly in the end plate has the advantage that there is even less electrical power loss, on account of the low-resistance junction. Moreover, in this way it is possible to produce even better thermal coupling of the diode to the end plate. As a result, the thermal resistance is reduced, allowing a higher coolant temperature or a higher barrier junction temperature of the diode. Furthermore, during the integration the necessary electrical leakage paths are formed in the radial direction. Compared to installation on the end plate, this leads to a further advantage in terms of volume and design, since in this case the leakage paths have to be formed in the axial direction. Finally, it is possible to dispense with a separate housing for the diode, which in addition to saving costs also reduces outlay on production.
If the fuel cell stack already has cooling ducts, it is at most necessary to provide an additional duct in the region of the diode. Usually, the feed and discharge lines for the cooling system are also formed via one or both end plates. If, in this case, the diode is arranged in the region of a feed or discharge line of this type, the need for an additional cooling duct is eliminated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.